Quote:
Originally Posted by ausastronomer
Hi Rod,
I am not quite sure why you need to consider this eyepiece, unless you are also changing other things.
Am I correct in assuming you own a 13mm ETHOS and a 20mm T5 Nagler?
If so, in your scope (20"/F5 FL = 2550mm) these eyepieces give you the following True Fields of View and magnification
13mm ETHOS = .51 degrees TFOV @ 196X
20mm Nagler T5 = .64 degrees TFOV @ 128X
If you purchase the 17.3mm DELOS with its 72 deg AFOV it will give you
TFOV = .49 degrees @ 147X
Consequently you gain nothing in terms of TFOV. It gives a slightly reduced TFOV compared to your 13mm ETHOS at lower magnification, thus contrast and detail will be slightly reduced compared to the 13mm ETHOS. I can understand however that 147X may be a more usefull magnification for you under variable seeing conditions than the 196X of the 13mm ETHOS, which may not always be useable.
I think in your case the 17mm ETHOS would be a better choice, but this then bumps heads with your 20mm Nagler in terms of TFOV.
Maybe you need a lengthy discussion with SWMBO and tell her you should sell the 20mm Nagler T5 and buy the 17mm and 21mm ETHOI to go with your 13mm ETHOS 
Cheers,
John B |
Hi John, thanks for your careful analysis of my EP situation.
I am aware of the various issues you raise, and have contemplated enlarging my Ethoi collection. Alas, I would prefer to spend my money on other things at this stage.
My interest in the Delos is not driven by what other EPs I may have in a similar focal length or TFOV, but rather the more intangible notion of "comfort". Part of this may be eye relief, but there can be other more subjective issues at play.
I adored my 22mm Panoptic and regret selling it. Notwithstanding its slightly inferior performance at edge of field, and narrower field of view (68 degrees) it was, for reasons which are perhaps a little hard to pin down, the most comfortable EP I have ever had. When I had it, it spent more time in the focuser of the Mary Rose than any other EP.
With my current line up, the 20mm Nagler plus Paracorr is my default observing choice. This renders an effective focal length of 17mm. While I love the 20mm Nagler, it does have fairly tight eye relief, which means that it is not optimally comfortable, and when glasses wearers share the views with me through the Mary Rose, it is definitely far from ideal for them.
I confirm that the 13 Ethos is not useful more often than not. The seeing in my area does not support 200x all that often. Having something in the 17mm area is a logical slot for me for times when the seeing is just not quite up to scratch for the 13E.
I am interested to know how the 17.3mm Delos may perform as an alternative. It might just be near dang perfect, for a pretty reasonable price that even SWMBO (aka the Minister for Finance) cannot argue with.
I am particularly interested to know how it performs without a Paracorr. If coma is an issue, then I will not be purchasing one.
